Spatially Determined Reactions 173 



amphipod Talorchestia longicornis, which moves toward the 

 light but comes to rest in shaded portions, seems to combine 

 positive phototaxis with negative photopathy (181). Loeb's 

 observations on the larvae of the arachnid Limulus, the horse- 

 shoe crab, and upon insect larvae, may also be mentioned 

 here. When strongly negative, the former moved away in 

 the line of rays of sunlight falling obliquely from a window 

 upon the vessel containing them ; the shadow of the window 

 bar lay across the vessel, and the animals continued to move 

 through it in the same direction, although, on passing out from 

 it, they went into a more brightly lighted region (239). A 

 similar illustration of phototaxis without photopathy was 

 found in the caterpillars of the Porthesia moth, which give a 

 positive response, and in fly larvae, which are negative (235). 



68. Direction and Intensity Theories of Phototaxis 

 The problem as to whether orientation to light is brought 

 about by the influence of the direction of light rays as such, or 

 by the fact that light falling upon an oriented organism from 

 a given direction affects symmetrical points with different 

 degrees of intensity, is one requiring much nicety of discrimi- 

 nation between concepts. Loeb, in his earliest discussion of 

 the subject, expresses himself positively in favor of the former 

 hypothesis. "The orientation of animals to a source of light 

 is, like that of plants, conditioned by the direction in which the 

 light rays traverse the animal tissue, and not by the difference 

 in the light intensity on the different sides of the animal" 

 (233). To this Bohn urges as a "fundamental objection" 

 that "the 'luminous rays' which strike a living body have, 

 save in wholly exceptional cases, various directions, being 

 reflected, diffused and refracted by neighboring bodies" (55). 

 Certainly if definite orientation to light occurred only when an 

 animal's body was traversed by rays in one predominant direc- 



